1. Field of the Invention
The present invention relates to a lens barrel for use with a projector, a camera or an endoscope and, more particular, to a lens barrel having a cylindrical spacer ring for regulating an axial distance between two lens components.
2. Description of Related Art
Practically, because of a strong demand for reduction in cost for a lens comprising a lens barrel and a lens system, it is usual to use plastic lens components for the lens system. Such a plastic lens component causes a greater change in dimension than glass lens components due to a change in ambient temperature and a change in ambient humidity. On the other hand, the lens barrel in which a lens system including a plastic lens component or plastic lens components are installed comprises a lens receiving barrel for receiving one or more plastic lens components, a lens holding ring for securely holding the plastic lens component in the lens receiving barrel and a cylindrical spacer ring for defining an axial distance of the plastic lens component from another lens component. These parts of the lens barrel are typically made of a metal which is less affected by a dimensional change due to a change in ambient temperature and/or a change in ambient humidity in order to be finished with high accuracy. If adjacent plastic lens components cause thermal expansion, the lens system encounters deterioration in optical performance due to a change in axial distance between the adjacent plastic lens components. Further, if firmly forcing the cylindrical spacer ring against a periphery of the plastic lens component, the remaining portion of the plastic lens component causes deformation and, in consequence, optical distortion, which leads to deterioration in its own optical performance. On the other hand, if loosely forcing the cylindrical spacer ring against a periphery of the plastic lens component, the plastic lens component is apt to become loose. In an attempt to solve the problems, there have been proposed various plastic lens holding devices. Japanese Patent Publication No. 3-32044 describes a lens holding device in which an elastic ring member is inserted between a plastic lens component and a lens holding ring so as thereby to absorb expansion of the plastic lens component. Unexamined Japanese Patent Publication No. 62-245209 describes a lens holding device in which a buffer ring having bits of elastic adhesive is positioned between a plastic lens component and another lens component or a stationary portion of a lens barrel so as to fixedly hold the plastic lens component. Further, Unexamined Japanese Patent Publication No. 5-188253 describes a lens holding mechanism in which a lens holding ring having tongue-shaped resiliently deformable claws is used to engage in recesses formed in a flange of a plastic lens component so as thereby to force the plastic lens component against a shoulder formed in a lens holding barrel. However, the use of elastic members for respective plastic lens components forming a lens system drives up costs. In addition, it is disadvantageous in light of a complex internal shape and a tapering bore to form a number of concave parts in a lens holding barrel for receiving plastic lens components therein. The lens holding barrel thus formed also drive up a manufacturing cost.
Consequentially, Unexamined Japanese Patent Publication No. 59-68710 describes a noble method for assembling a lens system into a lens barrel. In this method, lens components for forming the lens system are previously arranged in given axial positions using cylindrical spacer rings between respective adjacent lens components, respectively. The lens components is put in a metal mold for molding a lens barrel as they are remained in their axial positions and, subsequently, a molten resin is poured into the metal mold so as thereby to form a lens barrel with the lens components separately positioned by the cylindrical spacer rings therein as one whole.
However, in the case where a plastic lens component is fixed in position by a cylindrical spacer ring, a periphery or an edge of the plastic lens component against which the cylindrical spacer ring is forced expands in an axial direction upon a rise in temperature. This expansion of the plastic lens component entails a change in axial distance with respect to a lens component adjacent thereto as a necessary consequence, and hence a change in air space between the two adjacent lens components (which is defined as a distance between vertices of opposed surfaces of the two adjacent lens components). In consequence, since it is hard to make the optical performance of the lens system fall within tolerable limits due to changes in two optical factors, i.e. a change in axial distance and a change in air space, the mechanical feature in which a plastic lens component is forced against a cylindrical spacer ring, or vice versa, can not be employed in the conventional lens barrel.